It's possible to place widely used constants in an interface. If a
class implements such an interface, then the class can refer to those constants
without a qualifying class name. This is only a minor advantage.

The static import feature should always be
considered as a replacement for this practice.

Placing constants in an interface was a popular technique in the early days of Java, but now many
consider it a distasteful use of interfaces, since interfaces should deal
with the services provided by an object, not its data. As well,
the constants used by a class are typically an implementation detail, but
placing them in an interface promotes them to the public API of the class.